1. Field of the Invention
The present invention relates to a method of producing a lithium ion secondary battery comprising a nonaqueous electrolytic solution and more particularly to a method of producing and the structure of a safe lithium ion battery having a high charge-discharge efficiency comprising a low fluidity or gelled electrolytic solution.
2. Description of the Related Art
Portable electronic apparatus have found a very great demand for reduced size and weight. The accomplishment of the demand greatly depends on the enhancement of the properties of the battery to be mounted in these portable electronic apparatus. In order to meet this demand, the development and improvement of various batteries are under way. In particular, a lithium battery is a secondary battery which can realize the highest voltage, energy density and load resistance in the existing batteries. The improvement of lithium batteries is still under way.
FIG. 1 is a schematic sectional view illustrating the structure of an ordinary lithium ion secondary battery which has been put into practical use. The lithium ion secondary battery comprises as essential constituents a positive electrode 1, a negative electrode 2, and an ionically-conducting layer 3 provided interposed therebetween. In this lithium ion secondary battery, as the positive electrode 1 there is used a plate-like material prepared by applying a mixture of an active positive electrode material powder 1a such as lithium-cobalt composite oxide, an electrically-conducting powder 1b and a binder resin to a positive electrode collector 1c. Similarly, as the negative electrode 2 there is used a plate-like material prepared by applying a mixture of a carbon-based negative electrode active material powder 2a and a binder resin to a negative electrode collector 2c made of copper. As the ionically-conducting layer 3 there is used a separator made of a porous film of polyethylene or polypropylene filled with a nonaqueous electrolytic solution containing lithium ion. The battery structure of the present example comprises a single electrode laminate 4 having a separator laminated with an electrode.
The lithium ion battery comprising such a nonaqueous electrolytic solution is liable to rise in the danger of sparking, heat generation, etc. due to internal or external shortcircuiting caused by the rise in battery capacity. The rise in battery capacity faces a great apprehension that the battery might ignite. The elimination of this danger can be effectively accomplished by the reduction of the fluidity of the electrolytic solution.
However, since the lithium ion battery comprises a porous electrode formed by coagulating a particulate active material, it is very difficult for an electrolytic solution having a reduced fluidity to fill thoroughly microvoids in the electrode. On the other hand, it is necessary that the microvoids in the electrode be filled with an electrolytic solution to improve the battery properties.
Further, gelled electrolytes have been of interest and under extensive study for practical use from the standpoint of the reduction of the thickness of batteries. However, the gelled electrolytes, too, cannot be easily injected into the electrode. It is thus very difficult for the gelled electrolytes to fill thoroughly voids in the electrode. Batteries comprising a gelled electrolyte are disclosed in U.S. Pat. No. 5,460,904, xe2x80x9cNikkei Microdevicexe2x80x9d, Nikkei BP, August 1996, page 136, etc.
As mentioned above, it is not easy for any electrolytic solution to fill thoroughly microvoids in the electrode. Thus, the production of such a battery faces a problem that it is difficult to fill thoroughly voids in the electrode. This makes it impossible to provide a safe lithium ion secondary battery having a high charge-discharge efficiency.
The present invention has been worked out as a result of the inventors"" extensive studies of the filling of electrolytes.
An object of the present invention is to provide a production process by which a safe lithium ion battery having an excellent charge-discharge efficiency comprising a low fluidity or gelled electrolytic solution can be easily obtained and a lithium ion secondary battery having a structure that can enhance the charge-discharge efficiency thereof.
The first aspect of the method of producing a lithium ion secondary battery is a method, which comprises the steps of:
preparing an active material mixture by mixing an active material powder with a particulate polymer soluble in a nonaqueous electrolytic solution;
forming electrodes comprising said active material powder and particulate material by using the active material mixture as a raw material;
assembling said electrodes into an electrode laminate;
and then injecting said electrolytic solution into said electrode laminate.
The second aspect of the present invention is a method according to the first aspect of the present invention, wherein said particulate polymer soluble in a nonaqueous electrolytic solution comprises at least one of methacrylic polymer, acrylic polymer, polyethylene glycol, polypropylene glycol, and a copolymer obtained by the copolymerization of these polymers with other monomers.
The third aspect of the present invention is a method according to the first aspect of the present invention, wherein the method further comprises the step of introducing the particulate polymer soluble in a nonaqueous electrolytic solution externally into voids in said electrode before the step of assembling into the electrode laminate.
The fourth aspect of the present invention is a method according to the first aspect of the present invention, wherein the method further comprises the steps of:
coating said electrode with or dipping in a solution of said polymer soluble in a nonaqueous electrolytic solution;
and then drying before the step of assembling into the electrode laminate.
The fifth aspect of the present invention is a method according to the fourth aspect of the present invention, wherein said active material mixture further comprises a binder resin and the method further comprises the step of heating said electrodes at the temperature in which said particulate polymer is melt and said binder resin is not melt.
The sixth aspect of the present invention is a method according to the fifth aspect of the present invention, wherein said particulate polymer comprises at least one of polyethylene glycol and polypropylene glycol and the step of heating said electrodes is the step of heating them at 8020  C.
The seventh aspect of the present invention is a method according to the first aspect of the present invention, wherein a diameter of said particulate polymer is not larger than 20 xcexcm.
The eighth aspect of the present invention is a method according to the seventh aspect of the present invention, wherein a diameter of said particulate polymer is not larger than 5 xcexcm.
The ninth aspect of the present invention is a method according to the present invention, which comprises the steps of:
forming electrodes comprising an active material layer;
introducing a particulate polymer soluble in a nonaqueous electrolytic solution externally into voids in said electrode;
assembling said electrodes into the electrode laminate;
and then injecting said electrolytic solution into said battery structure.
The tenth aspect of the present invention is a method according to the ninth aspect of the present invention, wherein the step of introducing the particulate polymer comprises externally introducing a particulate polymer soluble in said nonaqueous electrolytic solution into voids in said electrodes which comprise active material layers formed of an active material powder to prepare an electrode comprising said particulate material in voids.
The eleventh aspect of the present invention is a method according to the tenth aspect of the present invention, wherein the step of introducing the particulate polymer is performed by supplying said electrodes into the particulate polymer and vibrating the particulate polymer.
The twelfth aspect of the present invention is a method according to the tenth aspect of the present invention, wherein the step of introducing the particulate polymer is performed by coating said electrode with or dipping in a solution of said polymer soluble in a nonaqueous electrolytic solution;
and then drying before the step of assembling into the electrode laminate.
The thirteenth aspect of the present invention is a method according to the tenth aspect of the present invention, wherein a diameter of said particulate polymer is not larger than 20 xcexcm.
The fourteenth aspect of the present invention is a method according to the thirteenth aspect of the present invention, wherein a diameter of said particulate polymer is not larger than 5 xcexcm.
The fifteenth aspect of the lithium ion secondary battery is a battery of the present invention, which comprises two opposing electrodes and a separator provided interposed therebetween, and a nonaqueous electrolytic solution retained in voids in said electrodes and said separator, wherein a gelling material is incorporated in said electrodes so that the viscosity or gelation degree of said nonaqueous electrolytic solution is higher toward said separator.
The sixteenth aspect of the lithium ion secondary battery is a battery of the fifteenth aspect, which comprises a plurality of electrode laminates.
The seventeenth aspect of the lithium ion secondary battery is a battery of the sixteenth aspect, wherein said plurality of electrode laminates are formed by alternately arranging a positive electrode and a negative electrode between a plurality of separated separators.
The eighteenth aspect of the lithium ion secondary battery is a battery of the sixteenth aspect, wherein said plurality of electrode laminates are formed by alternately arranging a positive electrode and a negative electrode between the gap of a wound separator.
The nineteenth aspect of the lithium ion secondary battery is a battery of the sixteenth aspect, wherein said plurality of electrode laminates are formed by alternately arranging a positive electrode and a negative electrode between the gap of a folded separator.